Linear Accelerators (also called “LINACS”) are widely used for a variety of tasks in a broad range of applications, including industrial applications such as Non-Destructive Testing (NDT), Security Inspection (SI), Radiotherapy (RT), electron beam processing —sterilization, and polymer curing, for example. Both accelerated electron beams, and Bremsstrahlung X-ray beam generated by such electron beams striking a conversion target at the end of an accelerating channel, are used for various tasks. The type of radiation beam selected is typically determined by the specific application and its requirements. In many applications, the requirements include energy variation and dose rate variation of the radiation beam, including broad RB energy variation, for example, from 0.5 MeV to a maximum energy, which typically does not exceed 10 MeV due to neutron production and activation problems. However, in some known cases, it can reach as high as 12 MeV, 15 MeV, 20 MeV, or even higher energies. Those familiar with the art are well aware that a linear accelerator is a sophisticated tool that does not always run efficiently, or does not perform at all over such a broad radiation beam operating energy range.
A linear accelerator includes a plurality of cavities, which gradually increase in length in the direction of the electron beam propagation to keep the particles in the right accelerating phase while their velocity increases. Once electron velocity reaches nearly the speed of light, the period of the structure and the shape of the accelerating cells usually remain the same until the end of the accelerator.
The front irregular section of the linear accelerator where electron velocities change substantially (from about 20% to 95% of the speed of light), and where the electrons are grouped together as a stream of bunches of electrons, is typically called the “buncher”. The buncher is responsible for forming the relativistic electron beam, which then enters the regular periodic part of the linear accelerator structure, called the “accelerator”, where the velocity of the electrons does not change substantially, while they reach higher energies above 1 MeV, and up to the N×10 MeV range or higher (where N is an integer 1, 2 . . . N).
An important parameter used for defining efficiency of the buncher is called “capture”, which presents a percentage of the particles captured by the accelerating fields, and synchronously accelerated to the required energy with respect to a total number of particles injected into the structure. Capture is very sensitive to the accelerating field distribution in the buncher. While one attempts regulating output energy of the produced radiation beam by varying input RF power into the linear accelerator, the structure of the fields in the buncher change, and the electron beam current in the accelerating channel may be reduced substantially due to degradation of capture in the buncher, thereby reducing intensity of the produced radiation beam.
The same may be true for regulating the radiation beam energy via switching of the injected electron beam pulse current without optimizing power and field distribution along the linear accelerator. The optimization is especially important for magnetron-driven linear accelerators, which represent most of the commercial markets. The optimization is even more important, for higher frequency linear accelerators designed to operate with an X-band power source, for examples, where lack of the input RF power generated by the best commercially available X-band magnetrons for a given task exists in most, if not all cases (so-called “power hungry” mode of operation).
An example of a standing wave linear accelerator known in the art is shown schematically in FIG. 1. The linear accelerator comprises a plurality of single RF cavities (not shown) coupled together in various ways depending on the RF structure design. RF power is provided by the RF power source 1, such as a magnetron or a klystron. The RF power propagates through an RF transmitting waveguide 2 and a high power circulator 3 to an input RF coupler 4, which is configured to match impedance of the external and internal RF circuit to minimize power reflections at the operating RF frequency. A high power circulator 3 prevents reflected power from propagating back to the RF source 1. The circulator 3 is called a “high power” circulator rather than a “low power” circulator because it is adapted for the maximum possible power generated by the RF source 1. Therefore, most of the RF power from the RF source 1 enters the linear accelerator.
In FIG. 1, the linear accelerator has two single RF structures coupled together, a standing wave buncher section 6 (or “buncher 6”) and a standing wave accelerator section 7 (or “accelerator 7”). The buncher section 6 contains a sequence of cavities, which are different in length to maintain proper phase shift between the accelerating fields in the neighboring cells to accommodate the gradually increasing electron velocity. The electron velocity rapidly increases to relativistic values (close to the speed of light) in the standing wave buncher section 6. Since the electron velocity becomes nearly constant in the accelerator section 7, all the cells have the same length. The RF source is powered by one or more sources (not shown), as is known in the art.
The single RF cavity of the input RF coupler 4 is also part of the linear accelerator RF structure. In the case of the standing wave linear accelerator, the input RF coupler 4 is usually placed somewhere after the buncher 5 and before accelerator 7, although it may be positioned anywhere along the linear accelerator. In the linear accelerator of FIG. 1, the buncher 5, the input RF coupler 4, and the accelerator section 7 together provide a single RF coupled accelerating structure of the linear accelerator. The RF power provided by the RF source is distributed among the linear accelerator cavities in accordance with the linear accelerator configuration and its RF properties, forming an RF field distribution for accelerating the charged particles, such as the electrons.
An electron beam 10 is formed in an electron gun 11, which can operate in a range of high voltages N×(1, 2, 3 . . . 100) kV, forming an electron beam 10 having a diameter small enough to enter the buncher 6. The electron beam 10 gains energy while propagating through the RF fields of the linear accelerator cavities of the buncher 6 and the accelerator section 7. After the electron beam 10 exits the RF accelerating structure, the electron beam is extracted outside the vacuum envelope of the linear accelerator through a vacuum-tight thin foil for electron beam applications, or it strikes a heavy metal target to generate bremsstrahlung (X-rays), as is known in the art. The election gun 11 may be a diode or triode election gun for example, as is known in the art. The electron gun 11 may be powered by the same power supply that powers the RF source or another power supply (not shown), as is also known in the art.
An optional external magnetic system 13, such as a focusing solenoid or a permanent periodic magnet (“PPM”) system, may be used. The magnetic system 13 may also include steering coils, bending magnets, etc., for correction of beam positioning inside the linear accelerator, or at its exit via electron beam window or conversion target 12. Use of an external focusing system is undesirable because it increases complexity and power consumption, and consequently increases the cost of the linear accelerator system. In standing wave linear accelerator systems, use of a magnetic system 13 can be avoided. In traveling wave linear accelerators, in contrast, a magnetic system 13 is provided in most cases, especially for the buncher portion of a linear accelerator.
To regulate energy in the standing wave linear accelerator of FIG. 1, which has a single RF feed from the RF source 1, field amplitude in the linear accelerator RF structure may be changed by varying beam loading or by varying input power regulation. Analysis of performance is shown in FIG. 2, which is a graph of Electron Beam Energy versus Peak Electron Beam Current (bottom axis) and Load Line and Dose Rate (top axis). FIG. 2 shows changes to a theoretical linear accelerator load line (squares) in a first approximation (Energy, MeV) to a corrected load line based on Parmela simulations of beam dynamics (diamonds). No external magnetic focusing field is provided. The graph of FIG. 2 also shows the corresponding dose rate curve (X's and triangles, respectively) based on the first linear load line (Dose Rate, R/min@1 m) and the other dose rate curve (or function) that corresponds to the load line based on Parmela calculations (Parmela/Dose). The effect of beam dynamics on output radiation beam characteristics is evident.
A reduced complexity and reduced cost linear accelerator is typically preferred. It is easier to design a standing wave linear accelerator to avoid use of the external focusing than it is to design a traveling wave linear accelerator without such focusing. While a traveling wave linear accelerator delivers some properties superior to those of a standing wave linear accelerator, it usually requires a focusing solenoid. A traveling waveguide principal behavior will be similar to that for the standing wave, described above.
Due to a common deficit of RF power, linear accelerators are usually designed for near maximum optimal output energy, where the dose rate is at its maximum defined by a well-known empirical ratio as follows:P=70×I×Wn,  (1)where: P is the Bremsstrahlung dose rate at 1 meter from a heavy metal conversion target, in R/min; I is the average electron beam current striking the target, in mA; W is the electron beam energy, in MeV; and n is a parameter that varies with energy (in several MeV range it is approximately 2.7).
For linear accelerators using an electron beam in a broad energy range, it is important to increase capture and efficiency at lower energy, thereby increasing the accelerated beam current and electron beam dose rate of the radiation beam. Where the linear accelerator is equipped with a conversion target to produce Bremsstrahlung radiation, the conversion dose rate is proportional to current, and nearly to a cube of energy. Consequently, lower energy operation of the linear accelerator at higher beam current becomes even more important. Efficient operation at lower energy is difficult to achieve, if the linear accelerator is designed to provide a beam at maximum energy at a given beam current to obtain the best radiation beam output.